This application claims the priority of German patent document 100 28 748.4, filed Jun. 10, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a power supply system for a motor vehicle having both a low-voltage onboard power supply and a higher-voltage onboard power supply.
A power supply system of this type is known, for example, from German Patent Document DE 197 24 356 C1. There, the voltage supply of 12 V or 14 V for low-voltage consuming devices corresponds to the low-voltage onboard power supply, and the voltage supply of 36 V or 42 V for high-power consuming devices corresponds to the higher-voltage onboard power supply. In such a power supply system, a generator normally generates the required voltage for the higher-voltage onboard power supply., while the voltage for the low-voltage onboard power supply is generated, for example, by a DC/DC converter. Particularly, the lighting and the electronic control equipment are powered by the low-voltage onboard power supply.
In such power supply systems (also called multivoltage onboard power supplies), a short circuit between the low-voltage onboard power supply and the higher-voltage onboard power supply would result in a rise of the voltage in the low-voltage onboard power supply such that the consuming devices of the low-voltage onboard power supply may be damaged or destroyed. It is also known that electromagnetically caused interferences occur to a higher degree in such multivoltage onboard power supplies.
It is therefore an object of the invention to provide a multivoltage onboard power supply with improved protection against short circuits and electromagnetic compatibility.
This and other objects and advantages are achieved by the power supply system according to the invention, in which it is checked, either by measuring the voltage at the low-voltage onboard power supply or by measuring the differential current in the high-voltage onboard power supply, whether a short circuit is occurring between the low-voltage power supply and the higher-voltage onboard power supply. And when a short circuit is detected, remedial measures are implemented to mitigate or eliminate the effects of the short circuit.
The term short circuit as used herein is understood to be a closed circuit, in that the voltage of the higher-voltage onboard power supply is applied to the consuming devices assigned to the low-voltage onboard power supply. For this type of short circuit, two couplings are required between the two onboard power supplies; first, a coupling of the plus connections and, secondly, a coupling of the minus connections of the two onboard power supplies.
In a first alternative, a short circuit is detected when the voltage of the low-voltage onboard power supply exceeds a value that is above the customary voltage value of the low-voltage onboard power supply. In a second alternative, a short circuit is detected when a differential current between the current flowing on the plus line and the current flowing on the minus line is present in the higher-voltage onboard power supply. The reason is that, in the no-fault normal case, the currents on the plus line and on the minus line are necessarily equal. Any deviation indicates a current which is flowing on a non-regular path and thus a possible line contact between the low-voltage onboard power supply and the high-voltage onboard power supply. In order to increase the reliability of the correct fault detection, the first and the second alternative may also be combined with one another.
An important aspect of the invention is also that the components of the higher-voltage onboard power supply either with respect to their minus connection or with respect to their plus connection are coupled by way of only one central bus line with the minus connection or with the plus connection of the low-voltage onboard power supply. This measure was found to be particularly advantageous with respect to the electromagnetic compatibility. Thus, with respect to one connection (preferably the minus connection), a first coupling between the low-voltage onboard power supply and the higher-voltage onboard power supply required for a short circuit is already provided intentionally. However, this does not result in a closed circuit such that the voltage of the higher-voltage onboard power supply is applied to the consuming devices of the low-voltage onboard power supply. For a (total) short circuit, a second coupling also has to be present between the respective other connections of the low-voltage onboard power supply and of the higher-voltage onboard power supply (that is, in the case of a preferably intentional coupling of the minus connections, also between the plus connections). Upon the occurrence of such a (total) short circuit as a result of a fault, according to the invention, the previously intentionally provided first coupling is separated by a controllable switch provided in the bus line. Thus, the effects of a short circuit can be effectively avoided.
As a result of the invention, on the one hand, the electromagnetic compatibility of a power supply system in the form of a multivoltage onboard power supply is increased and, on the other hand, the danger of the effects of a short circuit between the two onboard power supplies of different voltages is reduced.
In the case of such a multivoltage onboard power supply, the use of a DC/DC converter without potential separation is particularly advantageous, in the case of which DC/DC converter, the minus or ground connections of the different onboard power supplies are normally already interconnected anyhow. This coupling line of the minus or ground connections can simultaneously be used as a central bus line. However, according to the invention, the DC/DC converter must be modified with respect to the controllable switch in the bus line. This creates a particularly simple possibility for a central switch-off capacity in the event of a short circuit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.